It is important to keep the clearance between the tips of rotating blades and a static portion, such as the radially inner surface of an annular shroud, which surrounds the blade tips to a minimum. The clearance is controlled to minimise the leakage of turbine gases between the shroud and the blade tips. Minimising the leakage of the turbine gases improves the engine efficiency and thereby reduces the specific fuel consumption of the engine.
During the conventional operating cycle of a gas turbine engine the turbine blades, and the discs on which they are mounted, expand due to centrifugal forces acting on them as they rotate at high speeds and by thermal expansion due to being heated by the working fluid passing therethrough. The annular shroud however is stationary and only expands due to being heated by the working fluid. Differential expansion occurs and the clearance between the blade tips and the shroud has to be controlled to give a minimum clearance at steady state conditions whilst ensuring that the blade tips do not rub on the shroud during transients.
To this end, various control systems have been proposed. By way of example, GB2253012 discloses a blade tip clearance control apparatus which, as shown in FIG. 1, comprises a pressure tube 2 provided behind an arc shaped shroud segment 4. The pressure tube 2 is pressurised by a source of pressurised air to inflate the pressure tube 2 and thus move the shroud segment 4 closer to the tips of the blades 6.
The pressurisation is managed by a switch which is in turn controlled by a control of the engine. Similarly, U.S. Pat. No. 4,472,108 and GB1605255 also disclose pressure activated tip clearance control systems.
However, there are some instances where a gas turbine engine will be working to a higher than normal capacity (for example, during take-off of an aeroplane if the engine is an aero engine) and the blades will expand more due to the increased rotational speeds and/or temperatures encountered during such instances. Consequently, it is desirable to provide a tip clearance control system which can account for such instances. Furthermore, for the sake of reliability and ease of manufacture, it is desirable to have a tip clearance control system that manages the tip clearance independent of an engine control system.
The present disclosure therefore seeks to address these issues.